How to create a controllable moss material for a photoscanned object?
realistic is relative and increadibly difficult to achive when compared to photoscanned

you can use particles and get very realistic result, but a heavy performance hit

or you use vertex paint, or mask paint to mask out the areas where you want the moss to then procedually add the moss in the material editor with nodes. not very realistic, but still quick and easy

but the best would be to paint the textures, this will take the lowest ammount of performace but the highest ammount of work